In recent years, the long-sought goal of implantable biosensors has begun to see realization and, in some cases, clinical use. As this concept has seen continued research and development, issues regarding intracorporal fixation of the sensor have come to light. Particularly within blood vessels, the sensor is subjected to a continuous, pulsatile flow. This is a difficult environment in which to secure a sensor or other apparatus reliably without unduly restricting blood flow or impairing the vessel wall. One major vessel of interest in the realm of cardiology is the pulmonary artery. The pulmonary artery is a particularly challenging location in which to secure an intracorporal device because, in addition to the above considerations, the vessel is especially thin, compliant and prone to perforation.
Design considerations for an ideal fixation device intended for intravascular fixation are outlined as follows. The fixation device should be passive and maintain a separation distance between the sensor and the vessel wall to maintain blood flow past the sensor. The deployed size and radial strength of the device should be sufficient to prevent its migration into vessels that would be occluded by the dimensions of the sensor while creating minimal stress concentrations where the fixation device contacts the vessel wall. Alternatively, intracorporal devices can be designed sufficiently small in size so that when deployed in organs or regions with sufficiently redundant blood flow, the device can embolize on its own without harming the organ or the host. Finally, the fixation device should be sufficiently versatile as not to depend, within physiologically relevant ranges, on the size of the vessel in order to maintain its position.
There have been attempts to create devices intended to hold intracorporal devices fixedly within vessels. Several such attempts are described in patent publication number US 2004/0044393 and in European patent application number EP0928598. These attempts fall short of meeting all of the necessary requirements outlined above.
Prior art devices include a self-expansible stent on which an intracorporal device is mounted. This stent maintains a known length when implanted in a vessel where only the approximate diameter can be determined. Other devices and methods include fixation of a sensor in a bodily lumen, in which the sensor support is coupled to a fixation device. The fixation device is a stent or ring, has a sensor support coupled thereto and is intended to be sutured to the vessel wall or held in place by plastically deforming the structure using a balloon catheter. The ring is essentially a stent with an abbreviated length and suffers from the same shortcomings as traditional stent devices.
A stent is designed with mechanical characteristics that enable it to hold open diseased vessels post dilation. Therefore, the radial strength of the stent is greater than the inward radial forces exerted during vessel recoil. This primary requirement leads to a mismatch in compliance, with that of the stent dominating. Subsequently, stress concentrations are created at the interface of the stent and vessel. These stress concentrations are greatest at the terminal ends of the stent where there is an abrupt transition in stiffness between the stented and unstented segments of the vessel. Because undiseased vessels are usually more compliant compared to diseased ones, this compliance mismatch is amplified when placing a stent in healthy vasculature. Along similar lines, accurate stent sizing in the vessel is critical, especially in the case of the pulmonary artery. Accurate stent sizing to prevent migration and to avoid perforation of the vessel wall could be more difficult in healthy vasculature, especially the pulmonary artery, which has a smooth inner lining and greater compliance than most vessels. Thus, the physician must be conscious of the particulars of vessel compliance along with stent recoil and radial strength to choose the best stent expanded diameter for a given vessel. This determination presents its own set of challenges and requires an unnecessary increase in complexity, e.g., in deployment, and risk of complication. Therefore, the use of a stent to maintain an intracorporal device in a vessel is not optimal.
Thus, a need exists for devices and methods for fixing intracorporal devices which satisfy the design requirements described herein. Furthermore, a need exists to deliver and fix such devices in a safe, simple and predictable manner.